My own research on Steem: leptoquarks at the Large Hadron Collider

I had very little time to be present around Steem since half a year due to my research activities that were quite intense. As I am currently stuck at home due to the coronavirus, I finally got time to blog and start sharing those results.

^{[image credits: Steve Jurvetson (CC BY 2.0)]}
This post concerns my latest research article in which, with collaborators, we performed the most precise calculation of leptoquark production rates at the Large Hadron Collider (LHC).

However, before digging into this, let’s first discuss what leptoquarks are and why they are interesting beasts.

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Leptoquarks in a nutshell

In the Standard Model of particle physics, the structure of matter requires two classes of fundamental entities. Atomic nuclei are made of protons and neutrons, than are themselves made of quarks and antiquarks. In order to get to atoms, one additionally needs what we call leptons (electrons for instance). In the Standard Model, quarks and antiquarks interact with each other, as leptons do (via the known fundamental interactions).

There is however no option for one (anti)quark and one lepton to interact simultaneously.

This is where leptoquarks change the game: leptoquarks are hypothetical particles interacting together with exactly one lepton and one quark.

They are strongly motivated theoretically. They indeed appear, for instance, as soon as one tries to unify the fundamental interactions (see e.g. here), or when one invokes composite models (see e.g. here).

In the recent years, experimental facts have reinforced this motivation.

One of the fundamental properties of the muon, namely its anomalous magnetic moment, is one of the best measured quantity experimentally. At the same time, it is very precisely predicted.

Data and theory however disagree for several decades (see e.g. here), and leptoquarks are known to potentially save the day.

On the other hand, measurements of the decay properties of various B-mesons at several independent experiments have exhibited anomalies (see e.g. here). Once again, leptoquarks can cure the issue!

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Leptoquark production at the LHC

For the reasons sketched above, leptoquarks are widely searched for by the ATLAS and CMS experiments at the LHC. So far, there is no sign of them and limits on their viability are imposed.

Those limits however rely on theory predictions ignoring one important fact: leptoquarks can interact simultaneously with one quark and one lepton. Instead, only the fact they were strongly-interacting particles was accounted for.

This is what my latest article is about: the consistent inclusion of these ignored interactions in the predictions, and the improvement of their precision. This is illustrated in the figure below.

^{[image credits: arXiv]}
The x-axis consists in the leptoquark mass and the y-axis in the strength of the leptoquark interaction with quarks and leptons. The colour code represents the correct leptoquark production rate at the LHC: yellow means copious and blue means rarer.

If the leptoquark-quark-lepton interaction can be ignored, the dashed lines associated with a constant rate should be vertical lines. Our results however show this is clearly not the case, the impact being even larger when those interactions are very strong (upper part of the figure).

Interestingly, this is those are the relevant configurations in the light of the experimental anomalies.

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Take-home message: precision matters

Leptoquarks are hypothetical particles well motivated both theoretically and experimentally.

In my latest research article, we have achieved the most precise predictions for their production at the LHC, fixing issues ignored up to now. This will yield a better exploitation of LHC data, allowing for the extraction of more robust conclusions from the corresponding experimental searches.

^{PS: This article has been formatted for the steemstem.io front-end. Please see here for a better reading.}

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